This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2000-190624 filed on Jun. 26, 2000, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a flow amount control device, in particular, applicable to a flow amount control device that controls fuel amount to be supplied to a high pressure fuel pump in a common rail fuel injection system for a diesel engine (the diesel engine is hereinafter called an engine).
2. Description of Related Art
A common rail fuel injection system is well known as a system for injecting fuel to an engine. The common rail fuel injection system is provided with an accumulation chamber (common rail) commonly communicating with respective cylinders of the engine. A necessary amount of high pressure fuel is supplied to the common rail from the high pressure fuel pump whose fuel discharge amount is variable so that pressure of fuel accumulated in the common rail is kept constant. The high pressure fuel accumulated in the common rail is injected at a given timing to each engine cylinder from each injector that is connected to the common rail.
To keep pressure of fuel accumulated in the common rail constant, it is necessary to control flow amount of fuel to be supplied to the high pressure fuel pump and also to control flow amount of fuel to be discharged from the high pressure fuel pump according to engine operating conditions such as engine revolution or load.
The conventional common rail fuel injection system is provided with a fuel flow amount control device positioned between the high pressure fuel pump and a supply pump for delivering fuel to the high pressure fuel pump. The fuel flow amount control device serves to control flow amount of fuel to be supplied to the high pressure fuel pump and, thus, to control flow amount of fuel to be discharged from the high pressure fuel pump.
The conventional flow amount control device has an electromagnetic driving portion that drives a valve member according to a value of current applied thereto. A moving amount of the valve member varies in response to the value of current applied to the electromagnetic driving portion. Further, an area of opening formed in a valve body, through fuel passes to the high pressure fuel pump, varies according to the moving amount of the valve member slidably housed in the valve body. By controlling the flow amount of fuel that passes through the opening in the manner mentioned above, the flow amount of fuel to be supplied to the high pressure fuel pump is controlled.
However, since the opening of the valve body is formed in rectangular shape, the area of the opening through which fuel passes changes linearly in responsive to the value of current applied to the electromagnetic driving portion or the moving amount of the valve member. As a result, the flow amount of fuel to be supplied to the high pressure fuel pump and the flow amount of fuel to be discharged from the high pressure fuel pump vary linearly according to a value of engine load or engine revolution.
In a case that the opening area changes linearly in response to the moving amount of the valve member, a slight change of the moving amount of the valve member or a slight change of the opening area causes to change more largely the flow amount of fuel to be discharged from the high pressure fuel pump in an engine low speed region, compared with that in an engine high speed region since a time period during which the high pressure fuel pump sucks fuel is longer in the former region than in the latter region. Further, even if the engine revolution slightly changes in the engine low speed region, the time period during which the high pressure fuel pump sucks fuel and the amount of fuel to be sucked largely changes.
Accordingly, in the engine low speed region, the movement of the valve member affects largely on a change of the flow amount of fuel to be discharged from the high pressure fuel pump, causing to excessively increase or decrease fuel pressure in the common rail. As mentioned above, controllability of the flow amount of fuel to be discharged from the high pressure fuel pump is poor in the engine low speed region.
An object of the invention is to provide a flow amount control device in which a flow amount of fuel to be supplied to a high pressure fuel pump is adequately adjusted according to a value of engine revolution or engine load so that controllability of fuel amount of fuel to be discharged from the high pressure fuel pump is improved.
To achieve the above objects, in a flow amount control device for controlling flow amount of fuel to be supplied via a supply conduit to a high pressure fuel pump that discharges pressurized fuel to an accumulation chamber, a valve body has at least an opening for communicating with the supply conduit. The opening is composed of a first opening, a second opening whose circumferential length in the valve body is larger than that of the first opening, and a third opening bridging between the first and second openings in such a manner that the first, third and second openings are continuously formed in an axial direction of the valve body. A valve member, which is housed slidably inside the valve body, is provided inside with a fuel conduit through which fuel flows and in circumference with at least an outlet port connected to the fuel conduit. Driving means causes an axial movement of the valve member in the valve body when current is applied thereto.
With the flow amount control device mentioned above, an area of the opening communicating with the outlet port, through which fuel flows from the fuel conduit to the supply conduit, varies non-linearly in response to a moving amount of the valve member. That is, a change ratio of the area of the opening communicating with the outlet port to the moving amount of the valve member is variable and non-linear.
Accordingly, the change ratio of the area of the opening communicating with the outlet port to the moving amount of the valve member is smaller, when largeness of the area of the opening communicating with the outlet port is below a predetermined value, than that when the largeness of the area of the opening communicating with the outlet port is over the predetermined value. That is, a change ratio of the flow amount of fuel to be supplied to the high pressure fuel pump to the moving amount of the valve member is small in an engine low speed region and large in an engine high speed region.
As a result, controllability of the flow amount of fuel to be supplied to the high pressure fuel pump and controllability of the flow amount of fuel to be discharged from the high pressure fuel pump are improved in the engine low speed region. Further, the flow amount of fuel to be discharged from the high pressure fuel pump is sufficiently secured in the engine high speed region.
Preferably, the moving amount of the valve member changes in proportion to a value of the current to be applied to the driving means. In this case, the value of current to be applied to the driving means is controlled in response to engine revolution or engine load. The change ratio of the area of the opening communicating with the outlet port to the value of current applied to the driving means is smaller, when largeness of the area of the opening communicating with the outlet port is below a predetermined value, than that when the largeness of the area of the opening communicating with the outlet port is over the predetermined value.
Preferably, each shape of the first and second openings is roughly rectangular and shape of the third opening is trapezoidal. In this case, the flow amount of fuel to be supplied to the high pressure fuel pump varies in proportion to a change of the moving amount of the valve member in the engine low and high speed regions and varies smoothly along a quadratic functional line with respect to the change of the moving amount of the valve member in a transient region between the engine low and high speed regions.